Chesapeake Bay Shore Erosion

Within days after the storm, Baltimore
County surveyed its shoreline by plane. Erosion had occurred along roughly
3.5 linear miles, or 1.5% of the total length of the county's shoreline. Applying
that percentage to the length of shoreline
bordering Western Shore coastal counties, MGS calculated that approximately 32.7
miles of shoreline experienced
erosion during Isabel.
Two other variables needed to determine area and volume losses, shoreline retreat
and bank height, varied widely
fromsite to site. MGS assigned a value of 5 ft to both variables. That is, MGS
assumed that along eroded reaches, a 5-ft
high bank retreated 5ft. Based on that assumption, the area of fastland eroded
by Isabel equaled about 20 acres, and the
volume of eroded sediment, 4.3 million ft 3 (122,000 m 3). On average, those 20 acres,
lost in a single day, account for about
15% of the acreage lost from the Western Shore in any given year.
To convert the volume of eroded sediment to sediment mass, MGS multiplied sediment
volume (m3) by 1.30 metric
tonnes/m , the mean dry bulk density measured previously for Western Shore bluff
samples [12]. A total of 158,8000
metric tonnes of sediment were eroded during the storm.
Generally, when fastland sediments erode, only the finer-grained constituents
(silt andclay) remain suspended in the
water column. Coarser-grained sands and gravels form a lag deposit near the toe
of the bluff. The average Western
Shore bluff consists of nearly equal parts fine-grained (51%) and coarse-grained
(49%) sediments [12]. The fine-grained
fraction is of particular interest. Of the 158,800 metric tonnes of eroded sediment,
51%, or 81,000 metric tonnes, is the
estimated suspended sediment load contributed by storm-induced erosion of the
Western Shore. As a point of comparison, during Hurricane Agnes (1972),
a storm noted for torrential rainfall in the Bay watershed, the Susquehanna
River alone discharged over 31 million tonnes of suspende dsediment into the
Bay [13].

Seen
below, examples of sediment loss

MGS surveyed a reach of shoreline at the mouth of the Choptank R. a year before
and immediately after Isabel. Erosion rates varied. Todds Pt., ChoptankR., Dorchester
Co.